1. Field of the Invention
The present invention relates to a technique for restricting the use of content.
2. Description of the Related Art
A conventional mobile telephone with a packet communication function can download from servers on the Internet various content such as programs, image data, music data and so on. Although many of these contents can be used freely, some of them can't because of usage restriction set by the content supplier. For example, when delivering music data before its shipment for advertisement or a sample, it is preferable for content providers to be able to set a usage period for the music data; thus allowing the listener to play them until the usage period and prohibiting him from playing them after that.
To realize this, Japanese patent application Laid-open No. 2001-356966 filed on Jun. 14, 2000 discloses one method. In the method, when a server sends a content to a mobile telephone, the server adds to the header of the content information indicating the usage period. By this information, when the mobile telephone plays the content, the mobile telephone determines whether the time kept by its clock function passes the usage period.
However, the time of the mobile telephone is not always accurate, because some users of the mobile telephone may want to set the time from his own necessity; for example setting five minutes ahead of the correct time. In addition, when the user of the mobile telephone sets the time of the mobile telephone behind the correct time, the user can use the content that is overdue. To prevent this, there can be a following method. That is, when a mobile telephone processes a content, the mobile telephone receives a reference time from an external device such as Global Positioning System (GPS) satellites or a time server, and the mobile telephone compares the reference time and the usage period of the content.
However, the above method has a drawback in that when the mobile telephone processes the content, if it is situated in an underground or a tunnel where it cannot receive the reference time from GPS satellite or a time server, it cannot conduct a time comparison correctly. Further, for a method using a GPS satellite, mobile telephones have to be equipped with GPS receivers. Also, for a method using a time server, it is necessary to prepare and maintain the servers and other facilities. Both methods require additional cost.